From 2007-2012, I was a night shift package handler at FedEx.
When I was hired, my contract had a “guaranteed minimum” number of hours. 17.5 per week. If I clocked out early, I would still be paid for those hours with the caveat that you couldn't say no to your manager if they asked you to do some task and you were under your minimums. They did this to keep the ratio of packages-per-hour low, and I'm sure other less-than-altruistic corporate or tax benefits.
Then 2008 happened, and the company looked at how much they were paying people to not be there and they said that if you clocked out before you got 17.5, you were voluntarily refusing work, which was a condition of getting your minimums.
Friday at 930pm, I'd go to my boss and tell him I have X minutes left for my minimums, and he'd hand me a broom and tell me to sweep the warehouse, and stop when I hit my minimums. It was the most Sisyphus thing I've ever done.
Always look at everything, even perks, as to how they benefit the company.